Finding records with no associated records in rails 3
class Person < ActiveRecord::Base
has_many :pets
scope :with_dog, join(:pe开发者_如何学Cts).where("pets.type = 'Dog'")
scope :without_pets ???????????????????????????????????
end
class Pet < ActiveRecord::Base
belongs_to :people
end
I'd like to add a scope to the Person model that returns people who have no pets. Any ideas? I feel like this is obvious, but it's escaping me at the moment.
scope :without_pets, lambda { includes(:pets).where('pets.id' => nil) }
Try something like this:
Person.joins('left outer join pets on persons.id=pets.person_id').
select('persons.*,pets.id').
where('pets.id is null')
I haven't tested it but it ought to work.
The idea is that we're performing a left outer join, so the pets fields will be null for every person that has no pets. You'll probably need to include :readonly => false
in the join since ActiveRecord returns read-only objects when join()
is passed a string.
Mark Westling's answer is correct. The outer join is the right way to go. An inner join (which is what the joins method generates if you pass it the name/symbol of an association and not your own SQL) will not work, as it will not include people who do not have a pet.
Here it is written as a scope:
scope :without_pets, joins("left outer join pets on pets.person_id = persons.id").where("pets.id is null")
(If that doesn't work, try replacing 'persons' with 'people' -- I'm not sure what your table name is.)
You must use a LEFT OUTER JOIN in order to find records without associated records. Here's an adapted version of a code I use:
scope :without_pets, joins('LEFT OUTER JOIN pets ON people.id = pets.person_id').group('people.id').having('count(pets.id) = 0')
Im not sure if your pet model has a person id, but maybe this attempt helps you somehow
scope :with_dog, joins(:pets).where("pets.type = 'Dog'")
scope :without_pets, joins(:pets).where("pets.person_id != persons.id")
Update: Corrected the query method name from 'join' to 'joins'.
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